With Brad Pitt battling the living dead and government collapse in “World War Z,” out this weekend, it’s a good time to wonder who might be left among our leaders when they have to scrounge for calories, avoid the new predators on the block and go days without a smoothie.

If they’re super-fit and fat-free, are they better off when the dead rise?

What about President Obama? Thin, yes, but not in the way that makes you think he’d fall apart if he had to hunker down for a couple of days without a GNC JackedPack.

Zombies have stalked us on film for decades, reflecting contemporary pathologies: anxiety about war and race in the 1960s, when George A. Romero’s “Night of the Living Dead” trapped a group of strangers in a farmhouse; the obliviousness of modern culture in the seriocomic “Shaun of the Dead”; the isolation of the elite in “Land of the Dead” (Romero again).

More than other horror scenarios, zombies upend the natural order. The arrival of aliens requires either a technological fix (a fancy new weapon or, in the case of “Independence Day,” tricky software) or blind courage and luck, such as the disease that wipes out the Martians in H.G. Wells’s “War of the Worlds.” And there will never be enough vampires or werewolves to threaten the human race. Even at the top of the food chain, they are too reclusive, their attacks more deadly than infectious.

But zombism spreads fast, and a zombified planet challenges what it means to be human. It returns survivors to precivilization, and evolution goes to work: Those who remain will probably have roughly equal access to food and shelter, and similar abilities to fight or flee when needed. We won’t have to think about cardio vs. weights vs. yoga. We’ll all end up pretty sinewy and in need of a dental checkup.

But to survive in the long term means making it through those first critical weeks. Our natural state, after all, doesn’t include Whole Foods. We eat what we forage, grow or kill. Farming requires settling down. With zombies prowling, that’s too dangerous. Thus, back to hunting and gathering.

Who can best make the transition? Is our modern fixation on body fat and muscle tone actually a disadvantage in that environment?

“If you have no fat, you aren’t going to fare as well when zombies attack,” said Ranit Mishori, an associate professor of family medicine at Georgetown University. Studies of refugee populations and hunger strikers, she said, have pointed to the survival advantages of modest fat stores. Fat is the well of energy the body can draw on in times of deprivation, and as long as there’s a bottle of water to sip, people might get by for two weeks or so with little or nothing to eat.

The morbidly obese won’t do so well, since we’ll need to keep moving to escape the walking dead. But the ultra-slim and ultra-fit — even those as well-trained as marathoners or toting a bunch of muscle, which can be tapped for energy when calories are scarce — are likely to run out of gas before more moderately shaped people, which should be some comfort to most of us. Vitamin and mineral deprivation will produce chronic problems longer-term, Mishori said, but not in those first critical days.

My money is on Sen. Marco Rubio (R-Fla.) as first commander in chief of the Still Human States of America. A former football player, he has probably kept his underlying strength and is big enough to not get pushed around. Water-sipping aside, I imagine he has pudged up a bit since his college athlete days and, given his frame, could probably pack on a bit more to be safe.

As for the rest of us, we shouldn’t fret about that extra five or 10 pounds. We’ll need it.